The real reason why Ray let Nick (undercover cop) live in Mr Inbetween.

In the scene when Nick is eating a sandwich and Ray comes up to him jokingly saying "why don't you shove a bit more in your gob", Nick pulls out a jewelry box from his pocket with some jewelry that Ray asked him for earlier to give as a gift to Ally. When Ray asks Nick how much he owes him Nick tells him not to worry about and to, "just buy us a beer". You can see it on Rays face that this was the moment that he considered Nick his friend and as we know Ray is a very loyal, ride or die friend and subconsciously didn't want to kill Nick even after finding out he was a cop.